II-VI Materials Research Articles

Effects of multiple reflections on nonlinear absorption measurements.

Accurate measurement of nonlinear absorption coefficients by transmission-based techniques, such as Z-scan and its modifications, remains challenging due to high measurement errors. In this Letter, we claim that one of the reasons for that could lie in ignoring multiple reflections of light in plane-parallel samples in the presence of multi-photon excitation. Closed-form formulations for transmittance and reflectance coefficients, considering both linear and multiphoton absorption, are obtained in the geometrical optics approximation. The contributions of multiple reflections to the total transmittance are calculated for several II-VI materials reported in the literature. The results show a nonlinear dependence on incident intensity, ranging from 15.7% down to 7.9% for CdS, from 10.3% down to 5.8% for CdSe, and from 18.1% down to 9.1% for ZnSe. The dependence of the contribution of multiple reflections on the sample thickness is shown to exhibit a near exponential decay.

The Influence of Etching Method on the Occurrence of Defect Levels in III-V and II-VI Materials.

The influence of the etching method on the occurrence of defect levels in InAs/InAsSb type-II superlattice (T2SLs) and MCT photodiode is presented. For both analyzed detectors, the etching process was performed by two methods: wet chemical etching and dry etching using an ion beam (RIE-reactive ion etching). The deep-level transient spectroscopy (DLTS) method was used to determine the defect levels occurring in the analyzed structures. The obtained results indicate that the choice of etching method affects the occurrence of additional defect levels in the MCT material, but it has no significance for InAs/InAsSb T2SLs.

Ligand Steric Profile Tunes the Reactivity of Indium Phosphide Clusters.

Indium phosphide quantum dots have become an industrially relevant material for solid-state lighting and wide color gamut displays. The synthesis of indium phosphide quantum dots from indium carboxylates and tris(trimethylsilyl)phosphine (P(SiMe3)3) is understood to proceed through the formation of magic-sized clusters, with In37P20(O2CR)51 being the key isolable intermediate. The reactivity of the In37P20(O2CR)51 cluster is a vital parameter in controlling the conversion to quantum dots. Herein, we report structural perturbations of In37P20(O2CR)51 clusters induced by tuning the steric properties of a series of substituted phenylacetate ligands. This approach allows for control over reactivity with P(SiMe3)3, where meta-substituents enhance the susceptibility to ligand displacement, and para-substituents hinder phosphine diffusion to the core. Thermolysis studies show that with complete cluster dissolution, steric profile can modulate the nucleation period, resulting in a nanocrystal size dependence on ligand steric profile. The enhanced stability from ligand engineering also allows for the isolation and structural characterization by single-crystal X-ray diffraction of a new III-V magic-sized cluster with the formula In26P13(O2CR)39. This intermediate precedes the In37P20(O2CR)51 cluster on the InP QD reaction coordinate. The physical and electronic structure of this cluster are analyzed, providing new insight into previously unrecognized relationships between II-VI and III-V materials and the discrete growth of III-V cluster intermediates.

Colloidal InAs Tetrapods: Impact of Surfactants on the Shape Control.

We have approached the synthesis of colloidal InAs nanocrystals (NCs) using amino-As and ligands that are different from the commonly employed oleylamine (OA). We found that carboxylic and phosphonic acids led only to oxides, whereas tri-n-octylphosphine, dioctylamine, or trioctylamine (TOA), when employed as the sole ligands, yielded InAs NCs with irregular sizes and a broad size distribution. Instead, various combinations of TOA and OA delivered InAs NCs with good control over the size distribution, and the TOA:OA volume ratio of 4:1 generated InAs tetrapods with arm length of 5-6 nm. Contrary to tetrapods of II-VI materials, which have a zinc-blende core and wurtzite arms, these NCs are entirely zinc-blende, with arms growing along the ⟨111⟩ directions. They feature a narrow excitonic peak at ∼950 nm in absorption and a weak photoluminescence emission at 1050 nm. Our calculations indicated that the bandgap of the InAs tetrapods is mainly governed by the size of their core and not by their arm lengths when these are longer than ∼3 nm. Nuclear magnetic resonance analyses revealed that InAs tetrapods are mostly passivated by OA with only a minor fraction of TOA. Molecular dynamics simulations showed that OA strongly binds to the (111) facets whereas TOA weakly binds to the edges and corners of the NCs and their combined use (at high TOA:OA volume ratios) promotes growth along the ⟨111⟩ directions, eventually forming tetrapods. Our work highlights the use of mixtures of ligands as a means of improving control over InAs NCs size and size distribution.

Semiconductor Nanocrystals: Unveiling the Chemistry behind Different Facets.

ConspectusDeveloping next-generation colloidal semiconductor nanocrystals with high-quality optoelectronic properties and precise processability relies on achieving complete mastery over the surface characteristics of nanocrystals (NCs). This requires precise engineering of the ligand-NC surface interactions, which poses a challenge due to the complex reactivity of the multiple binding sites across the entire surface. Accordingly, recent progress has been made by strategically combining well-defined surface models with quantitative surface reactions to advance our understanding and manipulation of NC surface chemistry. Our lab has contributed to this progress by developing a size-dependent shape model of IV-VI NCs, gaining insights into their unique facet-specific chemistry, and developing a systematic ligand modification strategy for target applications. Furthermore, we have created well-defined facets in III-V NCs via a co-passivation strategy, addressing the previously lacking specific shapes.This Account is divided into three parts. First, we discuss the complexities involved in comprehensively understanding the nanocrystal surface structure at the atomistic level. We explain why we focused on well-defined NCs with a large exciton Bohr radius to explore facets, an essential aspect of surface heterogeneity across the entire NC. Second, we present our work on one of the most studied nanocrystals, IV-VI materials, and how facet-specific surface chemistry has led to a meaningful understanding and control of the NC's surface. We discovered a size-dependent facet distribution in IV-VI NCs and suggested facet-specific surface chemistry to improve the photophysical properties of NCs. We further modulate the electronic properties of NC assemblies for efficient optoelectronic applications. Third, we describe our recent success in achieving well-defined facets and their facet-specific chemistry in III-V NCs, which have yet to be explored as much as classical II-VI or IV-VI materials. We explain how controlling the surfaces in III-V NCs has been challenging. We present a precise growth platform for the geometric modulation of NCs, which can be further explored for shape-dependent exciton behavior and surface reactivities.Taken together, we present a compelling case for utilizing facet-specific chemistry as a platform for mechanistic investigation and morphology exploration, which can pave the way for developing high-quality and precisely designed NCs for optoelectronic technologies, unlocking new multidisciplinary applications.

Surface Engineering of Metal and Semiconductor Nanocrystal Assemblies and Their Optical and Electronic Devices.

ConspectusColloidal nanocrystals (NCs) are composed of inorganic cores and organic or inorganic ligand shells and serve as building blocks of NC assemblies. Metal and semiconductor NCs are well known for the size-dependent physical properties of their cores. The large NC surface-to-volume ratio and the space between NCs in assemblies places significant importance on the composition of the NC surface and ligand shell. Nonaqueous colloidal NC syntheses use relatively long organic ligands to control NC size and uniformity during growth and to prepare stable NC dispersions. However, these ligands create large interparticle distances that dilute the metal and semiconductor NC properties of their assemblies. In this Account, we describe postsynthesis chemical treatments to engineer the NC surface and design the optical and electronic properties of NC assemblies. In metal NC assemblies, compact ligand exchange reduces the interparticle distance and drives an insulator-to-metal transition tuning the dc resistivity over a 1010 range and the real part of the optical dielectric function from positive to negative across the visible-to-IR region. Juxtaposing NC and bulk metal thin films in bilayers allows the differential chemical and thermal addressability of the NC surface to be exploited in device fabrication. Ligand exchange and thermal annealing densifies the NC layer, creating interfacial misfit strain that triggers folding of the bilayers and is used to fabricate, with only one lithography step, large-area 3D chiral metamaterials. In semiconductor NC assemblies, chemical treatments such as ligand exchange, doping, and cation exchange control the interparticle distance and composition to add impurities, tailor stoichiometry, or make entirely new compounds. These treatments are employed in longer studied II-VI and IV-VI materials and are being developed as interest in III-V and I-III-VI2 NC materials grows. NC surface engineering is used to design NC assemblies with tailored carrier energy, type, concentration, mobility, and lifetime. Compact ligand exchange increases the coupling between NCs but can introduce intragap states that scatter and reduce the lifetime of carriers. Hybrid ligand exchange with two different chemistries can enhance the mobility-lifetime product. Doping increases carrier concentration, shifts the Fermi energy, and increases carrier mobility, creating n- and p-type building blocks for optoelectronic and electronic devices and circuits. Surface engineering of semiconductor NC assemblies is also important to modify device interfaces to allow the stacking and patterning of NC layers and to realize excellent device performance. It is used to construct NC-integrated circuits, exploiting the library of metal, semiconductor, and insulator NCs, to achieve all-NC, solution-fabricated transistors.

Kinetically controlled composition of III-V ternary nanostructures

Controlling the composition of ternary III-V and III-nitride nanomaterials such as vertical nanowires, horizontal nanowires, nanosheets, and nanomembranes grown by different epitaxy techniques is essential for band gap engineering and fabrication of nanoheterostructures with tunable properties. Herein, we investigate the diffusion-induced growth process of III-V ternary materials in different geometries including planar layers, nanomembranes, and horizontal and vertical nanowires grown by selective area epitaxy or with a catalyst droplet on top and derive a rather general equation connecting the composition of ternary solid with the composition of vapor. The form of this vapor-solid distribution remains identical for a wide range of geometries, while the coefficients entering the equation contain thermodynamic factors, kinetic constants of the material transport, and geometrical parameters of the growth template. General properties of the vapor-solid distribution are investigated with respect to material constants, growth condition, and geometry, including the interplay of thermodynamics and growth kinetics leading to the suppression of the miscibility gaps in InGaAs and InGaN systems. A good correlation of the model with the data on the compositions of InGaAs, InGaP, and AlGaAs materials grown by different methods is demonstrated. Overall, these results give a simple analytical tool for understanding the compositional trends and compositional tuning of III-V ternary nanostructures, which should work equally well for Si-Ge and II-VI material systems.

First-principles analysis of novel Mg-based group II-VI materials for advanced optoelectronics devices

Here, in this work, the structural, electronic, optical, and transport properties of Magnesium-based group II-VI materials are studied by employing the density functional theory calculations. The WC-GGA and EV-GGA generalized gradient approximations were employed to compute the structure parameters along with the electronic nature of the ternary-type materials. Our calculations confirmed the materials to be indirect band-type semiconductors with a bandgap value of 1.17 ​eV, and 0.51 ​eV, for the MgCdO2 and MgCdS2 materials respectively. Since Sulfur has a larger atomic radius than Oxygen, the bandgap value declines in going from MgCdO2 to MgCdS2 material. The computed total density of states for these materials displayed a sequential shift with increasing energy. The linear optical parameters like the real and imaginary parts of the dielectric function, the energy loss function, the absorption coefficient, the reflectivity, and the refractive index values are calculated and analyzed in detail. Moreover, the thermoelectric properties were also computed, and the results are presented in depth, suggesting all these materials to be efficient for thermoelectric device applications.

Transition metal-doped chalcogenide perovskite magnetic semiconductor [formula omitted

Diluted magnetic semiconductors (DMSs) are an important type of materials for spintronic applications. Conventional DMSs are typically based on transition metal doped III-V and II-VI materials such as GaAs and CdSe. In this work, we show that a new DMS material can be realized by transition metal doping in BaZrS3, a prototypical chalcogenide perovskite semiconductor. Mn and Fe elements are doped into the BaZrS3 lattice with a concentration of up to 5%. In particular, Fe-doped BaZrS3 exhibits room temperature ferromagnetism.

Fabrication of robust and highly stable Al2O3 passivated CdS anchored ZnO-Si nanowires: A new paradigm for hierarchical structure and sustainable solar fuel generation

We report sequential fabrication of a three-dimensional hierarchical structure of Al2O3-passivated-CdS-anchored high quality ZnO nanowire (NW) on patterned silicon NWs (Al/CdS/Si-ZnO HNWs) realized using Metal Organic Chemical Vapor Deposition for photoelectrochemical water splitting. Further, to improve PEC performance and photostability CdS anchoring and Al2O3 passivation were carried out. The optimally designed 60 CdS/Si-ZnO HNWs showed 25.8-fold (1.79%) enhancement in the photoconversion efficiency at 0.3 V vs Ag/AgCl compared to bare Si-ZnO HNWs (0.069%) along with 47.8-fold enhancement in the photocurrent density (1.42 mA/cm2 at 0 V vs Ag/AgCl). Further to improve the photostability (22.22 h) and reduce the photocorrosion, optimized 60 CdS/ZnO-Si HNWs were passivated using ultrathin Al2O3. Hierarchical assembly plays a key role in achieving the high photoconversion efficiency and photostability. The rationally designed hierarchical NW photoanodes synergistically improves photocurrent and photostability and may initiate facile integration of II-VI material with Si and could open the path for the next generation applications like energy harvesting as well as energy conversion devices.

All-laser-microprocessed waveguide Cr:ZnS laser

We report the proof-of-concept of an operational laser active medium with a depressed cladding waveguide manufactured in the volume of a Cr2+:ZnS single-crystalline sample and antireflection microstructures fabricated on its facets exclusively by femtosecond laser processing techniques. This allowed us to achieve transmittance in a broad range from 2 to 8 µm, approaching a maximum of over 90% near 2.5 µm, and lasing at 2275 nm at the average output power of 20 mW for the absorbed pump power of 500 mW with the slope efficiency of 5.5 %. This demonstration opens up a route towards the industrial fabrication of compact integrable laser sources and sensors based on II-VI materials.

Exciton radiative recombination time in a group III-V/II-VI core/shell quantum dot: Simultaneous effects of pressure and temperature

Excitonic properties of a compound semiconductor from group III-V and group II-VI materials are studied applying external perturbations. The simultaneous effects of pressure and temperature on exciton energies in an InP/ZnS quantum dot (core/shell) are found taking into account the ratio of radius of core/shell dot. The results bring out that the radiative lifetime enhances monotonically when the ratio of radius of core to shell quantum dot increases and it reaches a saturation value when Rc/Rs → 1 and the radiative recombination lifetime enhances upto 9 ns. The results can be applied for tailoring the excitonic optical transitions in nano-optical devices.

Clariant preserves profitability in first nine months of 2020 despite difficult economic environment: 3Q 2020

Here, in this work, the structural, electronic, optical, and transport properties of Magnesium-based group II-VI materials are studied by employing the density functional theory calculations. The WC-GGA and EV-GGA generalized gradient approximations were employed to compute the structure parameters along with the electronic nature of the ternary-type materials. Our calculations confirmed the materials to be indirect band-type semiconductors with a bandgap value of 1.17 ​eV, and 0.51 ​eV, for the MgCdO2 and MgCdS2 materials respectively. Since Sulfur has a larger atomic radius than Oxygen, the bandgap value declines in going from MgCdO2 to MgCdS2 material. The computed total density of states for these materials displayed a sequential shift with increasing energy. The linear optical parameters like the real and imaginary parts of the dielectric function, the energy loss function, the absorption coefficient, the reflectivity, and the refractive index values are calculated and analyzed in detail. Moreover, the thermoelectric properties were also computed, and the results are presented in depth, suggesting all these materials to be efficient for thermoelectric device applications.

Dislocation core energies of the 0° perfect, 60° perfect, 30° partial, and 90° partial dislocations in CdTe, HgTe, and ZnTe: A molecular statics and elasticity theory analysis

Although it is well known that dislocations degrade the electrical properties of devices based on II-VI materials, there is a profound lack of knowledge regarding these dislocations. Molecular statics simulations were used to study the 0° perfect, 60° perfect, 30° partial, and 90° partial dislocations in CdTe, HgTe, and ZnTe. The core energies were determined for the different possible core structures of these dislocations using the Stillinger-Weber potential. The results show that the 0° perfect dislocation is energetically more stable on the shuffle planes compared to the glide planes. For the other dislocations studied, the α configuration always has lower energy than the β configuration. The 60° perfect dislocation is energetically more stable on the shuffle planes for the α configuration, however, the opposite is true for the β configuration where it is more stable on the glide planes. The difference in energies between the single-periodic and double-periodic reconstruction was also investigated for the 60° perfect, 30° partial, and 90° partial dislocation on the glide planes. The Stillinger-Weber potential was only able to predict correctly the energetics for 30° partial dislocation with the α configuration, while the rest are in disagreement with previous ab-initio studies for semiconductor materials.

Optimization study of metallic sub-wavelength gratings as the polarizer in infrared wavelengths.

Despite the polarimetric detection in the infrared wavelengths of 8-10 µm being of great importance and broad applications, there has been limited addressing of the grating-based polarizers in this band. One of the main issues lies in the process incompatibility between the conventional nanofabrication technique and the II-VI materials such as HgCdTe, so that the direct integration of polarizers with sensors still remains a big challenge. This paper reports our recent work on optimizing the grating structures, materials, and nanofabrication processes for enhancing both the transmittance and the extinction ratio of polarizers on Si and/or ZnSe wafers, using numerical simulations for the grating design and electron beam lithography for the nanoscale pattern generation. By utilizing the finite-difference time-domain method, both the transmittance and the extinction ratio are maximized by optimizing the grating geometric dimensions and the duty cycle for two different grating materials of Al and Au for comparison. Based on the designed structures, nanofabrications of sub-wavelength gratings in both Al and Au are carried out, and the processes are compared for achieving high polarization performance. Optical characterizations of the fabricated polarizers demonstrate that both high transmittance and extinction ratio can be achieved in feasible parameters and the nano-process developed in this work.

Real colloidal quantum dot structures revealed by high resolution analytical electron microscopy.

The development of bright and photostable colloidal quantum dots has been a truly interdisciplinary feat. Designing a specific composition of core and shell materials and then producing the desired nanoarchitecture through chemical routes require a blend of physical and inorganic chemistry, solid-state physics, and materials science. In a battle to separate charge carriers from a surface wrought with defect states, complex shell structures with precisely specified gradient compositions have been engineered, producing nanosized emitters with exceptional stability and color purity. However, much of the success has resided in II-VI materials, such as CdSe, and progress is only just being made on cadmium-free quantum dots. This perspective will discuss the primary challenges in engineering colloidal quantum dots and highlight how the advent of advanced analytical electron microscopy is revealing the structure-function relationships of these complex systems.

Graphene-Assisted Epitaxy of Group IV Semiconductor Nanostructures and Thin Films

The growth of single crystalline group IV materials on amorphous substrates is useful in applications ranging from electronics to renewable energy. Graphene-assisted (or van der Waals) epitaxy is a promising route to achieve such films but has largely been limited to III-V and II-VI materials. In this work, we use graphene as a template for the low temperature epitaxy of group IV nanostructures and thin films. We investigate the effect of growth parameters and type of nucleation site (e.g., metal catalysts vs. graphene defects) on the crystal growth. We observe that metal catalysts enable in-plane group IV nanowire growth via the vapor-solid-solid mechanism at temperatures as low as 150 °C. The observation that neighboring nanowires are oriented 60º with respect to each other implies that the graphene crystal plays a critical role in the growth process. Graphene surface defects, induced by hydrogen radical exposure, can also be used to initiate crystal growth at elevated temperature. We leverage the differences in group IV atom sticking probability on graphene and amorphous surfaces to achieve selective epitaxy on graphene-on-oxide patterns. Transient studies as a function of the graphene pattern dimensions reveal a competition between crystal expansion and ripening. Our studies extend graphene-assisted epitaxy to a new, technologically-relevant materials system and provide fundamental insight into the underlying physicochemistry.

Directly diode-pumped, Kerr-lens mode-locked, few-cycle Cr:ZnSe oscillator.

Lasers based on Cr2+-doped II-VI material, often known as the Ti:Sapphire of the mid-infrared, can directly provide few-cycle pulses with octave-spanning spectra, and serve as efficient drivers for generating broadband mid-infrared radiation. It is expected that the wider adoption of this technology benefits from more compact and cost-effective embodiments. Here, we report the first directly diode-pumped, Kerr-lens mode-locked Cr2+-doped II-VI oscillator pumped by a single InP diode, providing average powers over 500 mW and pulse durations of 45 fs - shorter than six optical cycles at 2.4 µm. These correspond to a sixty-fold increase in peak power compared to the previous diode-pumped record, and are at similar levels with respect to more mature fiber-pumped oscillators. The diode-pumped femtosecond oscillator presented here constitutes a key step toward a more accessible alternative to synchrotron-like infrared radiation and is expected to accelerate research in laser spectroscopy and ultrafast infrared optics.

Revealing strong polytypism tendency in MgTe from first-principles

The search for stable polytype phases of III-V and II-VI materials has attracted great interest in recent years due to their great potential for a broad range of applications. Here using density functional theory, we systematically explore electronic structure properties of cubic (3C) and hexagonal (2H, 4H and 6H) polytypes of magnesium telluride: MgTe. Total energies and nearest-neighbor bilayer interaction energies evidence that MgTe shows a high propensity to polytypism. This finding is further supported by mechanical and phonon stabilities of all polytype phases of MgTe. Additionally, ground structural properties, energetic stability, mechanical properties, band structure energies, and dynamical properties are examined in relation with the stacking sequence of 3C, 2H, 4H, and 6H polytypes in the Mg-Te system.

Size-dependent bandgap and particle size distribution of colloidal semiconductor nanocrystals.

A new analytical expression for the size-dependent bandgap of colloidal semiconductor nanocrystals is proposed within the framework of the finite-depth square-well effective mass approximation in order to provide a quantitative description of the quantum confinement effect. This allows one to convert optical spectroscopic data (photoluminescence spectrum and absorbance edge) into accurate estimates for the particle size distributions of colloidal systems even if the traditional effective mass model is expected to fail, which occurs typically for very small particles belonging to the so-called strong confinement limit. By applying the reported theoretical methodologies to CdTe nanocrystals synthesized through wet chemical routes, size distributions are inferred and compared directly to those obtained from atomic force microscopy and transmission electron microscopy. This analysis can be used as a complementary tool for the characterization of nanocrystal samples of many other systems such as the II-VI and III-V semiconductor materials.